When the pulp is screened, a pulp suspension composed of fibres and water is fed to a screen comprising a screen cylinder provided with apertures, such as openings or slits of different shapes, and blades inside the screen cylinder, the blades rotating round the axle of the screen cylinder along the inner surface of the cylinder. In some cases, depending on the way of feeding the pulp to the screen, there are also blades rotating along the outer surface of the screen cylinder. The purpose of the blades is to maintain the inner or, correspondingly, the outer surface of the screen cylinder clean of the material that is screened and, on the other hand, to detach the fibre material already accumulated on the surface to be carried further in the screening process. In some cases, instead of rotating blades, a rotating screen cylinder is used, whereby cleaning is accomplished by the movement of the surface of the screen cylinder past a stationary blade. Nowadays, various multi-stage screens are further used comprising several successive screen cylinders in the axial direction of the screen. For cleaning the surface of the screen cylinder, either a separate blade by each screen cylinder or a one-piece blade extending over the whole screening surface can also be used.
In the screen, the pulp is typically screened over the whole height of the screen, whereby part of the fibres and water flows through the openings of the screen cylinder. As the screening proceeds, i.e. as the pulp flows downwards in the vertical direction of the screen, more water penetrates the screen cylinder in proportion to the amount of water initially in the pulp suspension. A problem arises as a result of this, since as the pulp consistency increases, the accumulation of the fibres on the surface of the screen also increases, thus impairing the screen operation and permeability.